Present casement or awning window assemblies include, between the glass panel and the rectangular frame in which it is mounted, a bulky structure consisting of wood parts and which is not easily dismountable should it be required to remove the glass panel from the frame. The wooden structure does not provide, in most cases, the desired insulation which it must maintain between both sides of the window assembly, especially in locations where important atmospheric differential exists, at certain time of the year, between the outside and the inside of a building.